The diagram (A) shows the activation of the receiver means of a prior art transponder if no carrier is detected. The receiver means are activated periodically in a listening mode during listening periods having a duration T1 and separated by a time interval T2. The duty cycle T1/T2 is relatively low to minimise the power consumption of the transponder.
If the transponder receives a message addressed to it, as shown in the diagram (B), the transponder detects the presence of a carrier in a listening period and maintains the receiver means activated to await the message and while receiving this particular message. On the other hand, if the carrier is an unwanted carrier, the diagram (C) applies. If the transponder receives a carrier during a listening period, it maintains the receiver means activated to await a message addressed to it. After a certain time, if no message that can be detected correctly by the transponder has arrived on the carrier (or, in a different embodiment, if the message is not addressed to the transponder), the receiver means are deactivated again after a time period T3 that is greater than the duration T1 of the diagram (A) situation. The receiver means are activated again either after a time interval equal to T1+T2 from the start of the listening period or after a time period T2 from the end of the listening period, as shown in the diagram (D).
Given that the duration T3 is greater than the duration T1, and generally significantly greater, in the case of receiving an unwanted carrier the duty cycle between the period of activation and the subsequent period of deactivation is generally greatly increased. In the situation of the diagram (A), the average consumption can be estimated at approximately 1.5 μA. On the other hand, in the situation of diagram (C) or (D), the average consumption may be approximately ten times greater, i.e. approximately 15 μA. Thus in the presence of an unwanted carrier, the ratio of the “ON” and “OFF” periods of the receiver means of a transponder of the type concerned is considerably increased, which greatly increases the power consumption of the transponder. This represents a major drawback, given that limited energy is stored in a transponder.
The object of the present invention is to alleviate the drawback mentioned above by proposing a method of managing receiver means of at least one transponder adapted to communicate at relatively long distance with at least one reader or sender, for the purpose of reducing the power consumption of the transponder in the presence of unwanted carriers in particular and, in one particular embodiment, also in the situation where a carrier does not carry a message addressed to the transponder concerned.
The expression “unwanted carrier” means a carrier coming from by a device, in particular a sender, foreign to the communication system concerned. In one particular embodiment, this expression also refers to a signal coming from any reader or sender that does not carry a message addressed to the transponder concerned. In some cases the transponder may consider a message addressed to it and received only partially or erroneously as equivalent to an unwanted carrier. Nevertheless, the transponder can also be adapted to detect, in some situations at least, that a message not received correctly is likely to be a message that is relevant to it and comes from a reader or sender of the communication system to which it belongs.